Dunaliella salina, a unicellular green alga with environmental tolerance, was employed as test organism to investigate the toxicity effects of a pesticide dimehypo. The influences of dimehypo on cell growth, single cell β-carotene level, cell morphology changes, and activities of superoxide dismutase (SOD) and catalase (CAT) were investigated. At the mass concentrations less than 0.5 mg·L^-1 dimehypo, cell responses were similar to control. When treated with 1.0-4.0 mg·L^-1 dimehypo, cell growth and single cell β-carotene levels declined at first and then revived. When mass concentrations were higher than 5.0 mg·L^-1 dimehypo, both cell growth and single cell β-carotene levels decreased until they were undetectable. The 10 d IC₅₀ of dimehypo on D. salina was 32 mg·L^-1. Dimehypo could stimulate the increase of CAT activity at a low concentration.